Bed removal

I need to replace the rear brake lines, and I plan on taking the bed off. Considering the truck is 13 years old, it wasn't very well taken care of, and that I live right in the middle of the rust belt, how likely is it that I will snap off every bolt trying to get the bed off?

Highly likely.
Try shooting some of your favorite penetrating oil on the bolts before you begin the project, if you've got time you might be able to hit them a few times.
When you begin the project get a map torch or even a propane torch, something you can use to heat things up with so you can hopefully loosen stuff up.
And last but not least I'd invest in heavy duty sockets, extension, breaker bar, and a cheater to put on the breaker bar.
Northern Hydraulics has a great line of impact sockets that can take a serious beating before they fail, they also have extensions and breakers. I'd probably find out what size sockets the bolts are and buy two or three of them so you dont get slowed down with a lack of tools dduring the project.

last year I removed the bed . the frame crossmember was rusted at the rear /over the fuel tank 2000 siL , 6 bolts came out not to hard. used a good quality penitrating oil a couple of days before attacking. did not use a gun . just a 1/2 drive with a long extension. you may have to work the bolt inand out on some a few times. do not break the body nut .I did not have to use the torch.

after the bolts are out lift bed up 1.5inches I used 2x4 's. then remove the frame bumper bolts and remove bumper. then you can slide back the bed . don't forget the fuel filler bolts ..

brake line to rear is easy to get to now. check for rusted frame/crossmembers. also rear shock bracket.

I just replaced my brake line going to the rear. I have a 2002 gmc sierra2500. I just fished the new line threw. I had a harder time getting the old one out. but i got it all back in the same hangers and everything. The hard part was getting the line out of the abs block under the cab. Its the line that is tucked in the back. (of course). Took me less time then pulling the body and messing with all of that or dropping the tank.Took about 2 hours from start to finish. I found it easy to cut the line just behind the nut and using a socket to get it out. The side on the abs block. And do what everyone else said and use penetrating oil. Let it site if you can. Will help alot.